Various devices used in the extraction of energy are known in the art using two basic mechanisms: these are variously denoted by; i) “drag” or “momentum transfer” or “momentum reversal” and ii) by “lift”. Known devices typically use one or other of these mechanisms. Momentum transfer systems rely on the fluid flow pushing against a vane, paddle or blade so that the vane is pushed in the same direction as the flow. A well-known example of a momentum transfer type device is the Pelton Wheel. However, during a full rotation cycle the cups produce significant drag on the return half of the cycle and therefore the efficiency is reduced. For this reason, such designs have not found favour for wind generation schemes, although their simplicity makes them ideal for use as anemometers for example where power efficiency is not an important consideration.
Another conventional means of transferring energy to a moving part is by use of the lift mechanism, such as the horizontal propeller blade turbine. With lift-type devices, the blade is impelled in a direction perpendicular to the direction of flow.
In the extraction of energy from a fluid or gas medium, a key parameter of importance and means of comparison of various methods is the efficiency with which energy is extracted. Efficiency is compared over a common area over which a mechanism intercepts the fluid. The efficiency in this case is defined as the ratio of the extracted power from the fluid flow over a defined area compared to the power available over the same area. The available power is proportional to the cube of the mean flow velocity and it can be shown that there is a fundamental limit to the amount of power per unit area that can be extracted from any medium flow. The efficiency used to compare different mechanisms is therefore defined as the power generated compared to that which would be theoretically available under the optimum load condition over an intercept area A.
In considering the force F perpendicular to the flow direction produced by flow against an inclined plate, the lift coefficient C1 is shown in FIG. 1a with dependence on incident flow angle, according to
  F  =            1      2        ·          C      l        ·    m    ·          v      2      
The maximum lift coefficient is about 1 and at large angles it approximates a sinusoidal function. At an angle of 90 degrees where the flow, designated by z in FIG. 1a, is parallel to the plate normal, the lift force is zero. The drag force defined as the force in the direction of flow is determined by the drag coefficient Cd according to
  F  =            1      2        ·          C      d        ·    m    ·          v      2      
FIG. 1b shows that the drag force is a maximum when the plate normal is aligned to the flow direction and varies approximately sinusoidally with incident angle, being zero when the plate surface normal is perpendicular to the flow direction. The maximum drag force coefficient of about 2 is about twice that of the lift coefficient for a thin plate. Thus, for power conversion, it is advantageous to include drag or momentum reversal effects, since the maximum efficiency factor is high. For a mechanism which utilises drag such as the Pelton wheel, the drag force is reversed over half the rotation and thus considerably reduces the efficiency. In lift-based designs the efficiency factor is optimised for a particular flow velocity by means of the blade incidence angle, therefore at low velocities and high velocities the efficiency is low. Consequently external means may have to be introduced to start the turbine particularly at low flow velocities. Vertical turbine types such as Savonius and Darrieus turbines are examples with low efficiencies at low flow velocities. Both the conventional horizontal and vertical rotating aerofoil blade turbines rely on lift. It is defined that lift forces act perpendicularly to the flow direction as in an aeroplane wing, whereas drag forces act in the direction of flow. The lift force depends on the attack (incidence) angle of the aerofoil blade, its area and its cross-section geometry. The section geometries are well understood and are characterised in the NACA numbered catalogue for their aerodynamic properties. They are typified by angles of attack between zero degrees to about twenty degrees of incidence to the flow, beyond which the wing is liable to stall. As above, the lift force can be expressed in terms of the lift coefficient C1:
  L  =            1      2        ⁢                  C        1            ·      ρ      ·              V        2            ·      A      where A is the aerofoil area, V is the flow velocity relative to the foil and ρ is the density of the flowing medium. Similarly, the drag force D can be expressed in terms of a drag coefficient Cd:
  D  =            1      2        ⁢                  C        d            ·      ρ      ·              V        2            ·      A      
Although drag mechanisms are not generally used to advantage in aerodynamics, for situations where the flow velocity is to be deliberately reduced, it is clear that from measurement of various plates that the drag mechanism can be about twice as efficient as the lift mechanism for producing utilisable force.
It is known to provide a vertical mill-type turbine in which each or several blades is rotatable about its respective axis, and also at a fixed distance (radius) about a common central axis, all these axes being parallel. Such devices include those disclosed in GB-A-2373028, JP-A-2004353637, EP-A1-1457672, BE-A-1013928, DE-A1-10123544 and FR-A1-2845428, GB 2241 747A, GB 2263 735A Such known turbines provide a simple linear relationship between the angle of each blade and the angle of rotation of that blade about the common central axis, to provide a blade rotation profile such as shown in graph FIG. 2a, denoted by the central line labelled 1, in which each blade turns through π radians during one complete revolution about the central axis. The linear relationship between the respective rotations is shown by the line 1 joining the origin (0,0) with points 3 and 4. In other words, after a complete revolution about the central axis, each blade will be in the same position as it was before, but rotated through 180°. The lift and drag forces on a blade and the angular definitions are shown in FIG. 2b. 
In addition to the strictly linear relationship between the rotation of the blade around its own axis and the rotation of the blade axis about a common central axis as cited, there is also defined by Goodden (GB 2241 747A, GB 2263 735A) that the blades may rotate counter directionally to the rotation around the common axis. It is also known that oscillation of the blade about its axis rather than complete rotation about its axis can be used to cause rotation of the central axis in a flowing medium. This is exemplified by Doering U.S. Pat. No. 5,324,164, Hamel U.S. Pat. No. 4,260,328, Unyushiyou JP55057672, Raymo EP0046122, Williams U.S. Pat. No. 4,618,312, Fork U.S. Pat. No. 4,380,417. However, these turbines suffer from inefficiency as the drag and lift contributions are not maximised, and cannot take into account factors such as blade interaction as discussed above.
The present considerations of full rotation of the blade in the same direction of the rotation about the common central axis therefore fall outside of the above.